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Circulator Pumps...Supply or Return?

Hello All,

I moved up to New England from Florida about six years ago. Until that point, my only experience with hot water boilers had been when I was in the Service, many, many moons ago.

Well, during my time up here I have heard that water circulator pumps should be installed on the return side of a hot water system and I've also heard that they should only be installed on the supply side.

The thing is - I haven't really heard a good explanation as to the Why of it, either way. I'm hoping that one of you will be able to give me an answer as well as a bit of insight about why it should be done one particular way and not the other.

Most you'll find on the return. Many boilermakers put them there from the factory though some are now packing loose. The "Wallies" that follow Dan Holohan's teachings "pump away" which is putting it on the supply pumping away from the expansion tank and fill. Less air problems that way.

I was always told the reason the Manufactures started putting them on the return was so the crate the boiler came in could be smaller. It wasn't supposed to stay there, the installers got lazy and left it there and it became the norm.

I was always told the reason the Manufactures started putting them on the return was so the crate the boiler came in could be smaller. It wasn't supposed to stay there, the installers got lazy and left it there and it became the norm.

Thanks for the replies guys. So it's because of air in the loops that the circ pumps should be on the supply side of things then? So is it easier to bleed the loops with them on the supply? or is it more of a prevention type thing, keeping them away from the water fill and exp tank? Thanks again for helping me out with this.

Hi - my 2 cents...
I did large building controls for quite a while, and once asked an old school engineer why hot pumps were always on supply - the hot side, to me, looked like a bad place for pumps. He had about 6 good reasons, I remember 2...

As I remember, never want to add pressure to a hot vessel - limit switches and such - so you always pull from a hot tank.

The other had to do with cavitation - which is really a pressure/gas expansion thing. You don't want to push air/bubbles into a hot tank.

That's what I remember - put I know all the larger systems I worked on pulled from boilers.

On commercial system they are almost always on the supply. On small residential systems you will find them on both the return or the supply. On residential you can normally get away with it, because not much head and not much height to a system. Are you better off pumping away, 90% of the time, "YES". Also check out the installation instructions, they almost always show the pump on the supply, even if it is packaged on the return.

Advantages of pumping away:
1) Oxygen will disolve proportionally with temperature and pressure. The high the pressure, the easier to suspend oxygen. The lower the temperature, the easier to suspend oxygen.
2) Head of pump will be added to sytem fill pressure. So with a 12psi fill pressure and a pump with 10 feet of head, you will have 16.33psi on the pump outlet and 12 psi on the inlet. With the pump on the return, you will have 12psi on the pump outlet and 7.67psi on the inlet. (This is not taken in any pressure drop between the fill and pump location.

Disadvantages of pumping away:
1) Will the pump adding its head to the fill pressure, any relief in front of it has to be rated higher than the fill pressure plus the head of the pump.
2) Pump will probably see high temperatures than being on the return. But with most new pumps this is not an issue.